The present invention relates to the detection of digital data contained in a composite video signal. More particularly, the invention relates to an apparatus for generating a detection clock signal for detecting digital data contained in the vertical blanking period of the composite video signal. Furthermore, the invention relates to a digital data detector which uses the detection clock signal.
In a character multi-broadcasting system, data corresponding to characters or figures are digitally encoded and inserted into the vertical blanking period of a composite video signal. Examples of such systems are a teletext system, the Korea Broadcasting Program Service (KBPS), and a caption system. The location of the encoded digital data within in the composite video signal is determined based on a sync signal (e.g. a clock run-in (CRI) signal) which precedes the data.
The digital data is extracted from the vertical blanking signal via a slicer. As illustrated in FIG. 1, a conventional slicer comprises an integrator 11 and a comparator 13. The integrator 11 inputs a CRI signal contained in the analog composite video signal and produces a slicing reference signal SR based on the CRI signal. The CRI signal is easily detected by the integrator 11 because it has a predetermined bit pattern. For instance, the CRI signal may comprise the 16 bit pattern "1010 1010 1010 1010".
The operation of the integrator 11 will be described below in conjunction with the waveform diagrams illustrated in FIGS. 2A to 2D. First, the integrator 11 inputs the composite video signal shown in FIG. 2A and a window pulse signal WP illustrated in FIG. 2C. Then, the integrator 11 integrates and averages the portion of the CRI signal which is input during a period that the window pulse signal WP is HIGH. Afterwards, the integrator 11 generates a slicing reference signal SR in accordance with the averaged value of the CRI signal.
As shown in FIGS. 2B and 2C, the window pulse signal WP goes HIGH during part of the period of the CRI signal. Preferably, the window pulse signal WP should be HIGH for a period greater than or equal to the period of three pulses of the CRI signal in order to precisely determine the slicing reference signal SR.
The slicing reference signal is illustrated in FIG. 2D. In the figure, the hatched portion indicates the period during the window pulse signal WP that the integrator 11 cannot determine the value of the slicing reference signal SR. Subsequently, when the reference signal SR is determined by the integrator 11, the reference signal SR is output to the comparator 13.
The comparator 13 also inputs the composite video signal and generates the sliced data S in accordance with the slicing reference signal SR. Specifically, the comparator 13 outputs the sliced data S when the analog composite video signal becomes greater than slicing reference signal SR. For example, as shown in FIG. 2A, the value of the reference signal SR is represented by the dotted line, and the comparator 13 outputs a HIGH signal as the sliced data S when the value of the analog composite video signal is greater than the reference signal SR and outputs a LOW pulse when the value of the analog composite video signal is lower than slicing reference signal SR.
FIGS. 3A to 3E illustrate the difference between a case in which the digital data is encoded in an ideal undistorted signal and a case in which the digital data is encoded in distorted signal. Specifically, FIG. 3A shows a composite video signal which does not contain a ghost and is not effected by noise. On the other hand, FIG. 3B shows a composite video signal which contains a ghost or is effected by noise. The position at which the digital data contained within the composite video signal is detected is shown in FIG. 3E.
In a conventional system, the position at which the data is detected is determined by clock signals generated in the receiver, and the clock signals are based on the duty cycle of the CRI signal contained in the composite video signal. Accordingly, when the ideal undistorted signal shown in FIG. 3A is input to the receiver, the comparator 13 outputs sliced data S having the waveform illustrated in FIG. 3C. Thus, the digital data reproduced by the receiver corresponds exactly to the original digital data transmitted in the video signal.
However, the transmitted television signal usually contains a ghost or other noise due to the characteristics of the multichannel broadcasting system. Accordingly, when the signal shown in FIG. 3A is transmitted, the receiver receives the distorted signal illustrated in FIG. 3B. Since the slicing reference signal SR is generated based on the CRI signal contained within the received signal, the waveform of the slicing reference signal SR is likewise distorted. Therefore, the comparator 13 erroneously outputs the sliced data S. For instance, the sliced data S may have a pulse width shown in FIG. 3D. As a result, since the receiver detects the digital data at the positions illustrated in FIG. 3E, the original data transmitted in the composite video signal cannot be precisely detected.